toplev.c (floor_log2_wide): Replace loop with faster bit operations.

toplev.c (floor_log2_wide): Replace loop with faster bit operations.

* toplev.c (floor_log2_wide): Replace loop with faster bit
operations.
(exact_log2_wide): Define in terms of the above.
* toplev.h (floor_log2): Use _builtin_clz family of builtins if
available.

From-SVN: r82778